Some countries actually make laws for the problem of hate speech against other people and usually many people get trapped when they say something about other people on social media. Hate speech is one of the factors that causes division between people and often this has a big impact when the problem becomes widespread and it is appropriate for everyone to avoid hate speech towards other people if they don't want to get in trouble with the law
You mean people are united only if the government imposes censorship and limits what can or cannot be said? How does hate speech cause division exactly? It's just a manifestation of peoples thoughts and opinions (however ugly they might be). And maybe division is not necessarily a bad thing. Would you like to be united with people you fundamentally disagree with and help them achieve their goals? Probably not.
Hate speech laws almost always evolve into some form of a censorship, when you no longer can express an opinion even if it's not target at anyone directly. Is publicly stating that e.g. you don't support gay marriage a hate speech? Who is to decide that?